ABSTRACT
INTRODUCTION Since the rst successes obtained in the second half of the twentieth century,1,2 human sperm freezing and embryo cryopreservation are today considered indispensable techniques available to all patients who utilize IVF therapy, such that they are routinely performed, and account for a large percentage of assisted pregnancies. By contrast, oocyte cryopreservation has been largely neglected, mostly due to the remarkable technical diculties related to its special cell structure and sensitivity. Nevertheless, since the rst human pregnancy reported from frozen eggs,3 considerable eorts have been made to improve on a suboptimal protocol. In this regard, the 2004 Italian Law No. 40 that regulates assisted reproductive technology (ART) is considered a key turning point. In fact, the limit of inseminating no more than three oocytes per IVF cycle, coupled with the legal prohibition against cryopreserving supernumerary embryos for reasons other than serious, documented, and not predictable adverse eects aecting women’s health, obliged embryologists to invest time and energy in developing eective cryopreservation methods.4 Fortunately, all of these eorts have ultimately been rewarded with excellent results,5-18 such that currently IVF clinics worldwide are able to enjoy the benets of this amazingly useful procedure. In fact, apart from restrictive legislation and/or ethical concerns, oocyte cryopreservation is of utmost importance in many aspects of ART, oering several valid solutions to clinical, logistical, and social problems. First of all, ovum donation programs allow a more convenient, exible, and cost-eective approach to the coordination between donor and recipient. Moreover, these programs can be precious tools for fertility preservation both for medical indications (e.g., cancer patients undergoing gonadotoxic chemotherapy/radiotherapy and eventual oophorectomy, and for women suering from premature ovarian failure arising from certain genetic disorders) or for social reasons (e.g., for preserving fertility in post-pubertal females without a partner, and to preserve fertility for women approaching menopause). Last but not least, oocyte cryopreservation may provide useful alternatives and advantages in infertility programs. In fact, it allows oocyte cryopreservation in the following cases: (i) collection of supernumerary eggs to keep for future use, thus avoiding the hormonal treatment necessary to sustain multiple follicular growth; (ii) risk of ovarian hyperstimulation syndrome, when ethical concerns and/or legal restrictions limit embryo freezing; (iii) as an alternative
option to embryo freezing to avoid potential entanglement in case of divorce and separation of couples; (iv) IVF procrastination due to no available/inadequate sperm sample; and (v) reported previous implantation failures with goodquality embryos.4,19,20
As already mentioned, oocyte cryopreservation protocols have evolved considerably over the years in terms of type and concentration of cryoprotectants used, such as cooling rates shiing from “slow freezing” to the more eective “vitrication” methodology. Briey, whereas slow freezing involves the gradual cell dehydration achieved with the combination of low cryoprotectant concentrations and slow cooling rates, the principle of vitrication relies on the initial exposure to high concentrations of cryoprotectant agents, followed by single-step ultra-rapid cooling in liquid nitrogen (−196°C) in order to achieve a solid glass-like state. e introduction of vitrication has dramatically increased cryopreservation eciency in terms of survival and, more importantly, pregnancy rates, such that it is today considered to be the method of choice to preserve both gametes and embryos, being nally freed of the restrictive label “experimental.”20
